1. Field of the Invention
The present invention relates generally to the field of optics, and more particularly to optical switches.
2. Description of Related Art
A transistor in the traditional sense has three electrical contacts for use as a switch, amplifier, or detector. The recent explosion in optical technologies for transferring information at light speed is resulting in a shift from electrical-centric networks to all-optical networks. Conventional wisdom dictates that the use of electrical transistors for switching in a system design is due to the historical belief that electrons and holes are more suitable to attain faster switching times than photons.
FIG. 1 is a prior art circuit diagram illustrating a Bipolar Junction Transistor (BJT) transistor 10. The electrical transistor 10 is constructed with three terminals, a base B 11a, an emitter E 12a, and a collector C 13a. The base B 11a serves as an input, while the emitter E 12a serves as a first output and the collector C 13a serves as a second output. Operationally, the electrical transistor 10 receives a small current that is applied to the base, represented by the symbol IB 11b with a positive phase 11c, which generates a first amplified replica of IB 11b as an emitter current with a positive phase 12c, IE, and generates a second amplified and inverted replica of IB 11b as a collector current with negative phase 13c, IC. A biased voltage Vcc 14 is typically placed at the collector terminal 13a, and a ground is typically placed at the emitter terminal 12a. 
Three defining characteristics make up the essential operations of the electrical transistor 10. The electrical transistor 10 produces a gain between an input and output. The change in the collector current IC 13b and the emitter current IE 12b are significantly greater than change in the base current IB 11b. This mathematical relationship can be represented by the equation, ΔIC=ΔIΔB, where β is typically a value between 20 to 100. Gain enables cascading large number of devices. Secondly, the electrical transistor 10 can switch fully ON or fully OFF, enabling the capability to design digital gates, e.g. NAND, AND, NOT, OR, and XOR. Thirdly, there exists an isolation in the electrical transistor 10 that produces an unidirectional signal flow. Unidirectionality means that the change in the collector current in response to applying an electrical current to the base occurs in such logical sequence, but not vice versa. For example, in the forward active mode where voltage is applied to the collector 13a, this results in little corresponding change in the characteristics of the base 11a. Therefore, the signal flow in the electrical transistor 10 is unidirectional, which flows only from the base 11a to the collector 13a, and not from the collector 13a to the base 11a. 
The isolation characteristic, in addition to the other two characteristics of gain and switching, are the essential features that enables the electronics chip industry to design and manufacture Very Large Scale Integrated (VLSI) circuits with millions of transistors. In designing a VLSI chip, the architecture is sub-divided into small linear sub-units or steps. Each subunit is built, analyzed, and tested separately as a unit. The sub-units then are pieced together as one large complex system. The underlying principle that enables a VLSI design is unidirectionality of signal flow. If the signal flow is bi-directional, when two sub-units are constructed in series, the entire system would require a complete re-analysis to test the functionalities of the system. Even though each sub-unit behaves as a single unit, multiple sub-units connected together as a large complex system do not operate as a single, easily predictable unit. If gain is present in the bi-directional subunits, then multi-state, and even oscillatory behavior with no functionally well-defined input or output may likely result. The bi-directionality of a large complex system hinders the ability to predict the performance based on single unit characteristics. Therefore, the unidirectional isolation characteristic of the electrical transistor 10 is a key-enabling feature that allows massive integration of transistors on a chip. The gain and switching feature allows for wide distribution and robust restoration of a signal while minimizing the effects of noise in the system. Prior research and development in the design of an optical transistor have resulted in slow switching time, a bulky device, and/or impractical implementation.
U.S. Pat. No. 5,748,653 discloses a device that possesses both elements for gain and switching. However, there are no means identified, implicitly or explicitly, for unidirectional signal flow. Care must be taken in this area since the performance of the device in an optical circuit will depend on the amount of isolation or unidirectional signal flow. If there is not enough isolation present, then unwanted oscillations and spurious behavior will result. Furthermore, the device with the stated amplifier disclosed in U.S. Pat. No. 5,748,653 is not linear or gain stabilized. Linear gain is an attractive feature if the device is operated in analog mode (i.e. output is not completely on or completely off but operated at an intermediate level) and a high fidelity output replica of the input is needed.
Accordingly, there is a need to have an optical transistor with fast switching time that is comparable or better than the switching time of an electrical transistor, possesses a definite means of isolation, and possesses linear gain if used in analog mode.